Preheat the oven to 400F.
Place your sliced potato pieces (I've directed to cut each potato into 8-12 cubes, as this will create more surface area to caramelize and reduce the roasting time) and your quartered onion in a bowl. Toss with 2 tbsp olive oil, rosemary, and approx 1 tsp salt and 1 tsp pepper.
Lay out on a baking sheet and roast till the surfaces are nicely browned, and the potato pieces just soft enough! This should take between 20-30 minutes.
Once cool, add to a blender or food processor with 4 cups of stock, and puree until smooth.
In a medium stock pot, heat 1 tbsp or so olive oil over med-high heat. Add the leeks and sauté till just golden and softened, about 5-7 minutes. Reduce the heat to medium. Add the wine if using, to deglaze the pot, and add a lovely bright crispness to the flavours. Simmer 2-3 minutes. Add a pinch of pepper.
Re-season, if needed with salt and pepper to taste. Finish with a final crack of fresh pepper.
Serve with savoury granola, crispy pancetta or grilled bread or croutons. I like one larger piece of crusty bread, lightly oiled and grilled briefly to get the crisp on. Then cover each piece with grated gruyere (or cheese of choice) and put back under the broiler to melt the cheese.
If you use vegetable stock, this is a lovely vegetarian meal. However if you'd like to use chicken stock, this is great as well. And if you want to add cubed cooked kielbasa or smoked ham pieces, this would definitely make the soup more hearty.
How about switching up the seasonings? I've used dill and thyme or tarragon in Spring, and classic rosemary and thyme in the Autumn and Winter. But if you sprinkled the potatoes with curry and cumin before roasting, and then adding another tsp of curry or ground coriander to the leeks, this would definitely be amazing as well.
The wine is optional, but will add a lovely flavour, and cut through the richness of the soup.
If you find that the soup is too thick for your liking, just add some stock to thin to desired consistency. This may happen if there are leftovers and you take the container out of the fridge.